Rain Water Chemistry

Compared to river and groundwater, rainwater has lower levels of many dissolved solids. Though the amount of any particular dissolved solid may vary from region to region depending on the proximity of the coastal or marine environment, rain water tend to be less alkaline than other fresh water.

Sulfur dioxide and nitrogen oxide emissions from power plants form acids in the atmosphere that fall to earth as rain, fog, snow or dry particles. This acid precipitation (known as "acid rain") is often carried hundreds of miles by the wind. Acid rain damages forests and causes lakes and streams to become acidic, killing the fish. Acid rain also damages buildings, historical monuments and even cars. The phenomenon of acid rain is well known but in India, acid rain is very rarely reported that too from such high industrial region as Chembur in Mumbai. Similar to surface water, rainwater quality also shows strong spatial and temporal variability.

Inland rain, that is the rain originating entirely over land or influenced to a large extend by landmass such as the northeast monsoon will have different chemistry compared to the southwestern monsoon starting from Kerala that is entirely ocean based. The major parameter in rain is generally Na and Cl suggesting to the overall strong influence of oceans on the chemical composition of rain. Other parameters may vary, SO₄ may be important in rain over urban areas such as Delhi due to thermal power plants and HCO₃^- and Ca may be important in rain over the Gangetic belt due to soil moisture evaporation and contribution from desert lands of Rajasthan. In fact evaporation over the plains of North India is so strong that rainwater chemistry here may be strongly linked to land than to the sea at least during the early stages of monsoon. Overall, rain carry a small amount of dissolved load but contributes significantly to the circulation of water by active participation in the hydrological cycle.

All water management scheme for better availability such as check dams and rainwater harvesting should recognise the interdependence of various sectors. Bicarbonate is the dominant ions in all types of water in the sub-continent among the cations. The sub-surface water is dominated by Na while the other two catagories, Ca is the dominant cation. Due to the large terestrial influence, it is not surprising that rain has similar proportions of constituents as in the surface water bodies in the region. The average ground water is quite saline and has very large proportion of all constituents including nitrates that may represent soil percolation of nitrogenous fertilisers extensively used in agriculture.

 Several attempts have been made to transfer water from region where it is surplus to the area that is deficient. Linking of river to do such water transfers no doubt will solve the water scarcity problems in acute areas but may also generate ecological imbalances. Solution of one problem should not lead to creation of another sets of problems.